1. Field of the Invention
The present invention relates to the area of filter technology and ceramics and concerns ceramic multi-layer filters and a process for their manufacture, like those that can be used, for example, to separate oil-water emulsions in metal cutting fabrication, to clarify beer, to purify gas, to split gas or to separate liquid-solid mixtures.
2. Discussion of Background Information
Despite the commercial successes of ceramic UF/MF membranes in recent years and the continuing growth of demand, a substantial disadvantage remains as compared to the much more prevalent polymer membranes: the relatively high price per filter surface.
Ceramic filter materials are normally built up of sintered-together particles whose interstices form the porosity. Obtaining the highest possible proportion of pore volume and the most uniform and closely distributed pore size distribution possible is required for filtration purposes. As a result, it is preferred for ceramic powder with a closely distributed grain size distribution to be used for manufacturing ceramic filter materials since these offer the best of the above-mentioned properties. Powders with closely distributed grain size distribution are known and standardized from the abrasives industry.
Normally, ceramic membranes are made of a multi-layer system of porous ceramic whose individual layers have different pore widths. The actual filtering layer is the thinnest and most micro-porous of the system. It is situated on a coarsely porous and thicker layer, and this in turn on the next layer, etc. The coarsely porous material forms the support, which simultaneously assumes the mechanic carrier function of the overall system and also frequently forms the filtrate collection structures. The intermediate layers between support and filtering layer serve to reduce the interstices between the coarse particles of the support and the support of the finer particles of the subsequent layer. Depending upon the desired size of separation, at least one layer, but most of the time at least two layers are currently applied on the support for micro-filtration membranes (size of separation 1000 nm to 200 nm), at least two, but for the most part more than three layers are applied on the support for ultra-filtration membranes (size of separation 100 nm to 10 nm) and more than three layers are applied on the support for nano-filtration membranes (size of separation less than 10 nm).
Manufacturing the above-mentioned ceramic membranes takes places by first forming, drying and firing the support, then the first layer is applied, dried and fired, then the next layer is applied, etc. until the layer made of the finest particles is applied, the layer formed is dried and fired. The sintering takes place in accordance with the degree of fineness of the coating with much lower temperatures than with the support.
The majority of the cost in manufacturing arises due to the multiple repetition of the cycle “coating, drying, sintering.” The thermal treatment steps within the process chain are already the most expensive as such so that multiple repetition increases this share immensely. In addition, a cost-intensive manual effort arises along with the other steps.
The joint sintering of ceramic layers made of different ceramics has been known as such for a longer period of time from the fabrication of ceramic multi-layer elements for applications in microelectronics. The term LTCC for “low temperature cofiring ceramics” was coined for this.
However, in this case, layers of different ceramics that have different properties (such as insulating and conductive) are sintered with one another with the goal of achieving the highest possible density of the layers (for example, U.S. Pat. Nos. 3,978,248 and 5,683,528).
On the other hand, in the case of ceramic filter elements, the most similar ceramic layers possible, differing from one another only in terms of their pore sizes, are supposed to be sintered jointly. According to the above-mentioned principle of manufacturing porous ceramics, this means that layers of the same ceramic, but with different degrees of grain fineness, must be sintered jointly.
The main problem with cofiring is the different sinter activity of variously fine powders as a result of difference volume/surface relations. As a result, coarse powders require very high temperatures for a stable grain—grain connection, which originates via surface diffusion or via evaporation or condensation mechanisms.
In the case of very fine powders, on the other hand, the sintering activity is so high that, with equally high temperatures as a result of volume diffusion, a strong densification takes place that is accompanied by grain growth. In this connection, the pore volume diminishes and the pore size distribution shifts in the direction of larger pores. This process is associated with a high volume shrinkage, while the grain—grain bond subsides in the case of coarse powders with lower shrinkage.
But even slight differences in the shrinkage of multi-layer elements with simultaneous sintering lead to a distortion of the multi-layer element or to internal strains that reduce the mechanical load-bearing capacity. In addition, the shrinkage itself is undesired since it leads to changes in the dimensions of the ceramic formed pieces that are difficult to reproduce and make expensive refinishing steps necessary in order to be able to comply with narrow dimensional tolerances.
According to WO 96/30207, a process is known in which the shrinkage adaptation of a component of a multi-layer system is achieved by the use of nanoscale powders. In the case of coarsely porous filters, coarse powders are used and the nanoscale powder is added to the mixture to promote its fusion, while, in the case of fine powders, the nanoscale powder itself is used and sintering inhibitors are added in order to prevent fusion that is too strong. Agglomerates of the nanoscale powder are also used as coarse powder.
Disadvantageous in the case of this process, however, is the fact that precisely coordinating the shrinkage of the individual components requires relatively expensive experiments, the processing of nanoscale powders is very expensive (for example in the case of dispersion), and the powders are very expensive. In addition, the mixing of powders with different degrees of fineness causes a reduction in the pore volume, which is undesirable for filter applications.
The variations for multi-layer filters cited in the exemplary embodiments mention shrinkages of 5% for the support and 4% for the layer, which leads to great problems in practical application.
According to WO 90/15661, a simultaneously sintered two-layer filter is known in which the sintering behavior of the support (called “membrane” in this case) is adapted to the sintering behavior of the layer (called “film” in this case) by a fine powder fraction being added to the coarse powder (4 nm up to 10% of the diameter of the coarse particles) and/or a sintering auxiliary agent being used in order to adapt the sintering temperature of the support to that of the layer.
In this connection, the problem also occurs, however, that the pore volume of the support is reduced by adding the fine powder fraction and coordinating the shrinkage of the layer to that of the support requires expensive experimentation. The difficulty of the process becomes clear in that the shrinkage of the film is supposed to be reduced additionally by high solid loading of the powder dispersion, which is particularly difficult to achieve in the case of fine powders. In addition, the sintering is conducted under pressure load in order to prevent distortion. The shrinkages of the overall system cited in the exemplary embodiments lie between 4 and 11%.
As a whole, two principle problems can be recognized that occur in the case of joint sintering of layers of different grain fineness with the same (low or high) temperature:    a) The different bond strengths of the powder particles i.e., in the case of high temperature, good bonding of the support, but strong sintering of the layer; in the case of low temperature, good formation of the layer but poor bonding of the support (as a consequence, for the purpose of avoiding these problems, every layer type is fired at different temperatures, which differ up to 1200 K).    b) The different shrinkage, which leads to crack formation and delaminations in the layer.
Problem b) leads to very special requirements being placed on layer structure in order to avoid cracks and delaminations. Thus, the following is cited in R. R. Bhave, Characteristics and Application, Van Nostrand Reinhold, New York, 1991:                The layers must be very thin (MF membranes between 25 and 50 μm, UF under 10, to some extent under 5 μm).        The roughness of the support should be low.        Significance is attached to the grain shape of the coating powder.        